94 BYZANTINE CHURCHES CHAP.
The nave is divided into two large bays of equal breadth but unequal length, the western bay being the shorter. In the latter the arches which support its roof are, to the east and west, semicircular, while those to north and south are roughly elliptical, springing from the same level and rising to the same height as the semicircular arches3 but being of shorter span. These elliptical arches extend to the outer walls of the church, thus partaking of the character of short barrel vaults.
Upon these arches is raised what has been called an elliptical dome. But in, no part has it the character of a true ellipse, nor does it spring from its supporting arches in the simple regular manner of a dome, but in the complex manner of a vault built upon arches of unequal curvature. It should therefore rather be called a domical vault. Where it shows above the roof it has the appearance of a modified and very low cone covering an irregular elliptical drum.
The eastern bay of the nave is square on plan, bounded by semicircular arches, all extended so as to form short barrel vaults. The western arch is joined to the eastern arch of the western bay, thus forming a short barrel vault common to both bays. The vault to the east runs to the semi-dome of the apse ; whilst the vaults to north and south, like the corresponding vaults in the western bay, extend to the outer walls and cover the eastern portions of the aisles and galleries. Above the supporting arches regular pen-dentives are formed, and above these there is a drum carrying a dome. The apse to the east of the nave is semicircular within and covered by a semi-dome.
Between that semi-dome and the eastern barrel vault of the nave a break is interposed, giving the bema arch two orders or faces, with their external and internal angles rounded off, and the whole surface of the semi-dome and of the bema arch is covered with mosaic. At one time the mosaic extended also over the surface of the barrel vault. The decoration in the semi-dome consists of a large cross in black outline upon a gold ground ; below the cro^s there are three steps set upon a double band of green that runs round the base of the semi-dome. A geometrical